Blow out main drain or not?

[ImpalaRob]

What you dont want is a pipe full of water at the freeze point, it will have no room to expand, then burst the pipe. A pipe with some water will most likely support itself and the ice within will form to the shape of the pipe.......... preventing damage. If you can blow it out, do it and avoid the possible costly repairs.

[JimK]

What you dont want is a pipe full of water at the freeze point, it will have no room to expand, then burst the pipe. A pipe with some water will most likely support itself and the ice within will form to the shape of the pipe.......... preventing damage. If you can blow it out, do it and avoid the possible costly repairs.

Yes, I do understand the potential for damage if the water in the pipe freezes. It seems in our area that is not happening, as evidenced by my not blowing out the main drain pipe since the pool was installed over 8 years ago and not having any issues. Don't know if I've just been lucky or if its just not necessary where I live. Perhaps just for extra insurance I will start blowing out the main drain line when closing just in case we have an unusually cold winter.

I suspect that blowing out any of the lines below ground/water level is overkill where I live. The times we have had ice in the pool, it has been a very thin layer and doesn't last. I really doubt the lines below the water/ground level are ever exposed to sub freezing temps. That said, should I really take the chance? Being wrong would be very expensive!

I do have a couple question though about blowing out the main drain line.

There is a valve that I can close that stops water flow from the main drain. I've read here that others blow out the line then close the main drain valve to prevent the water from refilling the pipe (air lock effect). Is the main drain valve really air tight (I'm skeptical)? If it isn't really air tight, then water will just refill the pipe again.

Also, it seems to me that forcing the water out of the main drain pipe requires a bit more air pressure than blowing out the returns since the main drain is in the deepest part of the pool (much more water pressure to work against). Do I risk damaging anything applying higher air pressures to force the water out?